The invention relates to a resistance paste consisting of a polymer binder in which there are dispersed electrically conductive particles, solvents and additives. Further, the invention relates to a resistance layer produced from such a resistance paste.
There are known polymer thick-film resistance pastes having a thermoplastic polymer as binder (cf.iee productronic, vol. 29, 1984, No. 1/2, pages 35/36). Printed layers made from such a resistance paste can be cured at low temperatures. Under the influence of temperatures exceeding 80.degree. C., however, the resistance value is no longer stable. The resistance value increases as the temperature rises, until the electrical conductivity of the layer is interrupted as a result of the melting of the binder. Such layers are unsuitable for processing, e.g. by way of the reverse laminating process (cf. German Application DE-OS 33 22 382), as the generation of heat and pressure necessary for relaminating the layer from an intermediate member onto the final substrate causes the binder component to soften and smear the layer.
There are also known resistance materials having a duroplastic binder component (cf. iee productronic, vol. 29, 1984, No. 1/2, pages 35/36). Their resistance value decreases as the curing temperature or the curing time, respectively, increases until excessive thermal influence brings about the destruction of the binder in the layer due to thermal decomposition.
The electric stability of such resistance layers may be better than that of thermoplastic resistance layers. A disadvantage, however, is the high curing temperature between 180.degree. C. and 230.degree. C. required in such cases, as the number of possible substrates to be applied to the layer is limited due to the high curing temperature.
German Application DE-OS 21 07 424 describes a layer containing phenolic resin and melamine resin.
German Patent DE-PS 31 48 680 describes a binder having melamine resin, alkyd resin and epoxide resin as components. The useful life of a resistance layer produced therefrom is prolonged by adding a vinyl chloride copolymer. This vinyl chloride copolymer reduces the friction coefficient of the resistance layer, so that the latter is only slightly stressed by the sliding movement of a resistance layer tap.
It has been found that resistance layers made from the known resistance pastes are not resistant to various organic substances like fuels and oils. In such cases of utilization, the resistors would have to be protected by expensive sealing materials.